1. Technical Field
This invention relates to electronic devices used to shape amplitude with respect to frequency, and more particularly, to bandpass filters using Generalized Impedance Converters (GIC).
2. Discussion
Linear circuit functions such as bandpass filters are used to control or shape the time or frequency domain of high frequency signals. At first, passive elements, namely resistors (R), inductors (L) and capacitors (C) were used. It was later found that if active elements such as operational amplifiers are used, then the reactive elements (capacitors and inductors) may be removed without loss of circuit performance. A likely element to replace was the inductor because of its size, power distribution, non-linearity and weight. While capacitors may be directly included in integrated circuit realizations using silicon layering techniques, inductors generally must be realized by discrete elements that must be externally "wired" into the integrated circuit.
Generalized Impedance Converters were developed to overcome some of these problems. These circuits simulate the frequency response characteristics of inductors by using a combination of operational amplifiers, capacitors and resistors. It should be noted that by using impedance scaling, a Generalized Impedance Converter can also simulate a capacitor. Even though capacitors are used in the design of the Generalized Impedance Converter, a trade-off for more efficient capacitor values can be made. The uses of Generalized Impedance Converters were limited though because the operational amplifiers are active devices and, consequently, one terminal must typically be grounded. Thus the RLC network must also be of the type with all simulated inductors and simulated capacitors grounded which limits the application to highpass filter circuits.
Critically coupled transformers use coupled capacitors or coupled inductors to magnetically couple the input to the output. Magnetically coupled circuits pass signal voltage to the output at frequencies near resonance and block other frequencies. The circuit model used to describe coupled inductors uses three inductors in a "T" shaped configuration with two of the inductors floating and one grounded. An analogous model exists for capacitors. If the impedance on both sides of the coupled transformer is matched, the output will have symmetric upper and lower stopband attenuation. The pass band width at resonance depends on the degree of coupling between the circuits. Again, the problem with using inductors in this circuit design is that it is not easy to implement in integrated circuit form, and it has high power dissipation.
Current technology is lacking a simple realization of an active bandpass filter with symmetric upper and lower stopband attenuation, a variable pass band width, and variable attenuation. It can be appreciated that there is a need for simulated coupled inductors in critically coupled transformers for electronic filtering circuits, especially in those realized in integrated circuit form.